An isolated boost power converter comprises a magnetically permeable multi-legged core (102) comprising first and second outer legs (132; 136) and a center leg (134) having an air gap (138) arranged therein. A boost inductor (Lboost) is wound around the center leg (134) or the first and second outer legs (132; 136) of the magnetically permeable multi-legged core (102). The boost inductor (Lboost) is electrically coupled between an input terminal (104) of the boost converter and a transistor driver (106) to be alternatingly charged and discharged with magnetic energy. A first and second series connected secondary transformer windings (SW1; SW2) with a center-tap (116) arranged in-between are wound around the first and second outer legs (132; 136), respectively, of the magnetically permeable multi-legged core (102). In a first discharge state, the magnetic energy stored in the boost inductor (Lboost) is discharged by directing a discharge current from the boost inductor through a primary transformer winding (PW1; PW2) and in a second discharge state, the magnetic energy stored in the boost inductor (Lboost) is discharged by discharging a magnetic flux through the first and second secondary transformer windings (SW1; SW2).
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1. An isolated boost power converter, comprising:
a magnetically permeable multi-legged core comprising first and second outer legs and a center leg having an air gap arranged therein;
an input terminal for receipt of an input voltage, Vin;
a boost inductor being wound around the center leg or the first and second outer legs of the magnetically permeable multi-legged core;
the boost inductor being electrically coupled between the input terminal and a driver to be alternatingly charged and discharged with magnetic energy;
the driver having a driver output coupled to a primary transformer winding wound around the first and second outer legs of the magnetically permeable transformer core;
the driver being configured to apply a primary voltage to the primary transformer winding in accordance with a driver control signal;
first and second series connected secondary transformer windings with a center-tap arranged in-between and wound around the first and second outer legs; respectively, of the magnetically permeable multi-legged core; and
a rectification circuit electrically coupled to respective outputs of the first and second secondary transformer windings to provide a rectified converter output voltage, Vout;
wherein
in a first discharge state, the magnetic energy stored in the boost inductor is discharged by directing a discharge current from the boost inductor through the primary transformer winding; and
in a second discharge state, the magnetic energy stored in the boost inductor is discharged by discharging a magnetic flux through the first and second secondary transformer windings.
2. An isolated boost power converter according to
3. An isolated boost power converter according to
4. An isolated boost power converter according to
during the first discharge state, magnetically decoupling the boost inductor from the first and second secondary transformer windings to deliver the magnetic energy to the primary transformer winding;
during the second discharge state, magnetically coupling the boost inductor to the first and second secondary transformer windings through a shared flux path in the magnetically permeable multi-legged core.
5. An isolated boost power converter according to
6. An isolated boost power converter according to
7. An isolated boost power converter according to
the primary transformer winding comprises first and second series connected half-windings arranged around the first and second outer legs, respectively;
the first and second series connected half-windings having an identical number of windings; and
the first and second series connected secondary transformer windings have an identical number of windings.
8. An isolated boost power converter according to
9. An isolated boost power converter according to
the boost inductor being coupled between the input terminal and a first transistor driver output coupled to a first end of the primary transformer winding; and
the second boost inductor being coupled between the input terminal and a second transistor driver output coupled to a second end of the primary transformer winding.
10. An isolated boost power converter according to
11. An isolated boost power converter according to
the center-tap is electrically connected to a negative rectified converter output voltage or the rectified converter output voltage, Vout; and
the respective outputs of the first and second secondary transformer windings are coupled to an opposite output voltage relative to the center-tap voltage through first and second rectifying elements.
12. An isolated boost power converter according to
a second magnetically permeable multi-legged core;
a second boost inductor magnetically coupled to a center leg of the second magnetically permeable multi-legged core to store magnetic energy therein,
the second boost inductor being electrically coupled between the input terminal and a second driver to be alternatingly charged and discharged with magnetic energy;
the second driver having a second driver output coupled to a second primary transformer winding wound around a first outer leg and a second outer leg of the second magnetically permeable transformer core;
the second driver being configured to generate a second primary voltage to the second primary transformer winding in accordance with the driver control signal; and
first and second secondary transformer windings wound around the first outer leg and the second outer leg, respectively, of the second magnetically permeable multi-legged core, wherein:
the first secondary transformer winding of the second magnetically permeable multi-legged core is coupled in series between the rectification circuit and an output of the first secondary transformer winding of the magnetically permeable multi-legged core; and
the second secondary transformer winding of the second magnetically permeable multi-legged core is coupled in series between the rectification circuit and the output of the second secondary transformer winding of the magnetically permeable multi-legged core such that:
in the first discharge state, the respective magnetic energies stored in the first and second boost inductors are discharged by directing respective discharge currents from the respective boost inductors through the respective primary transformer windings; and
in the second discharge state, the respective magnetic energies stored in the respective boost inductors are discharged by discharging respective magnetic fluxes through the respective first and second secondary transformer windings.
13. An isolated boost power converter according to
14. A method of generating a rectified converter output voltage, Vout from an input voltage, Vin, by an isolated boost power converter according to any of the preceding claims, comprising steps of:
generating a pulse width modulated driver control signal;
supplying the pulse width modulated driver control signal to the driver;
gradually increasing a duty cycle, D, of the pulse width modulated driver control signal from below 0.5 to a value above 0.5; and
adjusting the duty cycle, D, to a desired value to reach a desired or target AC or DC voltage at the rectified converter output voltage, Vout.
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The present invention relates to an isolated boost power converter comprising a magnetically permeable multi-legged core comprising first and second outer legs and a center leg having an air gap arranged therein. A boost inductor is wound around the center leg or the first and second outer legs of the magnetically permeable multi-legged core. The boost inductor is electrically coupled between an input terminal of the boost converter and a transistor driver to be alternatingly charged and discharged with magnetic energy. A first and second series connected secondary transformer windings with a center-tap arranged in-between are wound around the first and second legs, respectively, of the magnetically permeable multi-legged core. In a first discharge state, the magnetic energy stored in the boost inductor is discharged by directing a discharge current from the boost inductor through a primary transformer winding and in a second discharge state, the magnetic energy stored in the boost inductor is discharged by discharging a magnetic flux through the first and second secondary transformer windings. In this manner, the first and second secondary transformer windings may replace the traditional separate flyback winding utilized for start-up purposes of isolated boost power converters.
Isolated boost power converters are generally accepted as a highly efficient converter topology or architecture for high power converters with low input voltage and high output voltage. Isolated boost power converters are very useful for DC-DC voltage conversion in a diverse range of applications such as fuel cell converters, electric vehicles applications and avionic applications. However, a disadvantage of prior art isolated boost power converters is the need for a so-called flyback winding during a start-up phase or state of the power converter. During start-up, a duty cycle of a Pulse Width Modulated (PWM) control signal applied to a driver circuit must be ramped-up slowly to avoid excessive in-rush currents. During ramp-up of the duty cycle, it starts at a value much less than 0.5 which means that the driver circuit is placed in an open or cut-off state during a cycle of the PWM control signal without any low impedance path to a positive or negative input voltage terminal or rail. This situation leads to excessive voltage spikes across the boost inductor(s) which spikes may exceed the rated break-down voltage of semiconductor devices, such as MOS transistors, of the driver circuit so as to destroy these. This problem has previously been addressed by adding a flyback winding and a flyback diode to the isolated power converter providing a discharge path for energy stored in the boost inductor. However, the addition of a flyback winding has numerous drawbacks as the flyback winding is a separate power transferring element that is relatively costly, adds to size and increases component count of the boost power converter.
A first aspect of the invention relates to an isolated boost power converter comprising a magnetically permeable multi-legged core comprising first and second outer legs and a center leg having an air gap arranged therein. An input terminal is adapted for receipt of an input voltage, Vin. A boost inductor is wound around the center leg or the first and second outer legs of the magnetically permeable multi-legged core where the boost inductor is electrically coupled between the input terminal and a driver to be alternatingly charged and discharged with magnetic energy. The driver has a driver output coupled to a primary transformer winding wound around the first and second outer legs of the magnetically permeable transformer core and the driver is configured to generate a primary voltage to the primary transformer winding in accordance with a driver control signal. First and second series connected secondary transformer windings with a center-tap arranged in-between are wound around the first and second outer legs, respectively, of the magnetically permeable multi-legged core and a rectification circuit is coupled to respective outputs of the first and second secondary transformer windings to provide a rectified converter output voltage, Vout. In a first discharge state, the magnetic energy stored in the boost inductor is discharged by directing a discharge current from the boost inductor through the primary transformer winding. In a second discharge state, the magnetic energy stored in the boost inductor is discharged by discharging a magnetic flux through the first and second secondary transformer windings.
The ability provided by the present isolated boost power converter to discharge magnetic energy stored in the boost inductor through the first and second secondary transformer windings provides a novel solution to start-up of isolated boost power converters/DC-DC converters allowing these to operate below 50% duty cycle, D, of a Pulse Width Modulated (PWM) driver control signal. Consequently, the output voltage range at Vout can be extended down to zero without utilizing the traditional separate flyback winding because the first and second secondary transformer windings provide the functionality of the separate flyback winding of prior art boost power converters. The omission of the traditional separate flyback winding leads to a significant simplification of electric and magnetic circuit design, reduces component count, improves reliability, reduces the price and size of the power converter and decreases manufacturing costs. Furthermore, power conversion efficiency during start-up, and for operation below 50% duty cycle of the PWM driver control signal, is improved without affecting normal boost operation because copper material is not wasted on the traditional flyback winding. It is also noticeable that secondary transformer winding normally is designed for low winding resistance such that the additional flyback winding functionality requires no modification of an existing secondary transformer winding. The normal low resistance of existing secondary transformer windings also means that its use as flyback winding in accordance with the present invention can increase power efficiency compared with the traditional separate flyback winding. Furthermore, the second discharge state also allows stored magnetic energy in the boost inductor to be discharged or dissipated during error states of the isolated boost converter which e.g. may be caused by switching errors in the driver or an output short circuit condition.
The skilled person will understand that the term “isolated” does not imply that the input and output sides of the present isolated boost power converter necessarily are galvanically isolated by the magnetically permeable multi-legged core even though they may be so in numerous embodiments of the invention. Respective negative terminals or rails of the rectified output voltage Vout and the input voltage Vin may be electrically coupled to each other, for example through a shared GND connection such that a galvanic connection is established between the input/primary and output/secondary sides of the isolated boost power converter.
According to a preferred embodiment, during the second discharge state the first and second secondary transformer windings are coupled in parallel from the center-tap arranged between first and second series connected secondary transformer windings to the rectified converter output voltage, Vout. This allows discharge current to be directed to the rectified converter output voltage by both half-windings so as to minimize conductive losses in the secondary transformer winding. The magnetic flux flowing through the first and second secondary transformer windings, generated by the magnetic energy stored in the boost inductor, is converted to respective discharge currents flowing through the first and second secondary half windings SW1 and SW2. Thereby, power or energy is transferred to the rectified converter output voltage so as to control the latter when the output voltage is below a minimum voltage required for normal boost mode operation.
The present isolated boost converter is preferably configured such that the second discharge state is automatically entered in response to a reversal of magnetic flux rate in the boost inductor. The reversal of magnetic flux rate may be caused by the driver entering a non-conducting or OFF-state for example when a duty cycle of a pulse width modulated driver control signal is less than 0.5. Under these conditions, a resulting drop in charging current flowing into the boost inductor will cause the reversal of the magnetic flux rate. The driver may for example enter the non-conducting state when all transistors of the driver simultaneously are in non-conducting states. The driver may comprise a half-bridge or an H-bridge with two or four MOS transistors, respectively. The H-bridge or full-bridge transistor driver may have first and second complementary driver outputs coupled to respective ends of the primary transformer winding. When all transistors of the half-bridge or full-bridge transistor driver are placed in non-conducting states by the pulse width modulated driver control signal, a voltage across the driver rapidly rises to a destructive level due the continued effort of the boost inductor to force current towards the driver. This undesired rise of boost inductor voltage takes place during start-up of the isolated boost converter where the duty cycle D of the pulse width modulated driver control signal must be initialized to a value below 0.5, preferably close to zero, to avoid large in-rush currents. However, the automatic entry into the second discharge state provided by this embodiment eliminates the generation of destructive boost inductor voltage spikes by discharging the magnetic energy stored in the boost inductor as the discharge currents running through the first and second secondary transformer windings. The automatic entry of the second discharge state may be caused by the above-mentioned reversal of the flux rate through the boost inductor due to a sudden decrease of boost inductor current. The decrease of boost inductor current may be caused by the off-state entry of the driver or by an error condition.
In accordance with a preferred embodiment of the invention, the boost converter is configured to change between the first and second discharge states by selectively coupling and decoupling the boost inductor from the secondary transformer windings such that:
As previously mentioned the first and second secondary transformer windings are preferably configured to discharge the magnetic energy stored in the boost inductor by supplying a discharge current to the rectified converter output voltage, Vout, so as to transfer energy to the output. In this scheme, the first and second secondary transformer windings act jointly as a flyback winding.
According to an advantageous embodiment of the invention, the magnetically permeable multi-legged core comprises:
In yet another embodiment of the invention which comprises the center leg with the air gap and the first and second outer legs, the primary transformer winding comprises first and second series connected half-windings wound around the first and second outer legs, respectively, of the magnetically permeable multi-legged core. Preferably, the first and second series connected half-windings have an identical number of windings and the first and second series connected secondary transformer windings likewise have an identical number of windings.
In this embodiment, the first half-winding of the primary transformer winding and the first secondary transformer winding are both wound around the first outer leg of the magnetically permeable multi-legged core. Likewise, the second half-winding of the primary transformer winding and the second secondary transformer winding are both wound around the second outer leg of the magnetically permeable multi-legged core to achieve good magnetic coupling between the primary winding and second winding.
The skilled person will understand that the present isolated boost power converter may comprise many different primary side circuit topologies in addition to the previously mentioned half-bridge and full-bridge drivers. In one embodiment, the primary side comprises a first boost inductor and a second boost inductor such that the first boost inductor is coupled between the input terminal and a first transistor driver output. The first transistor driver output is coupled to a first end or first winding output of the primary transformer winding. The second boost inductor is coupled between the input terminal and a second transistor driver output. The second transistor driver output is coupled to a second end or second winding output of the primary transformer winding. The first and second transistor driver outputs may comprise respective drain or collector terminals of a MOS or bipolar transistor. The use of at least two boost inductors is advantageous because this reduces driver component count for example by reducing the number of semiconductor switches that may comprise respective transistor switches.
In another embodiment, the boost inductor comprises a first half-winding and a second half-winding of the primary transformer winding to provide an integrally formed boost inductor and primary winding which leads to improved copper utilization. In this embodiment, the magnetic energy stored in the boost inductor is directly transferred to the first and second secondary transformer windings by a magnetic flux through the magnetically permeable multi-legged core. In the above-discussed embodiments with a separate primary transformer winding and boost inductor, magnetic energy is initially stored in the boost inductor and subsequently released or discharged, during the first discharge state, as discharge current flowing through the primary transformer winding to induce a primary side voltage therein.
In a number of embodiments of the isolated boost power converter a rectifying element is electrically coupled to the center-tap to conduct a discharge current, during the second discharge state, from the first and second secondary transformer windings to the rectified converter output voltage, Vout. The discharge current is induced by the magnetic flux generated by the boost inductor flowing through the first and second secondary transformer windings. The rectifying element is preferably electrically coupled to a predetermined electric potential of the boost converter such as a power supply rail, including ground, a negative DC supply rail or a positive DC supply rail, of the secondary side of the isolated boost power converter. The rectifying element may be required if the rectification circuit comprises a full-bridge rectifier or a voltage doubler because during the first discharge state, the center-tap voltage differs from both the rectified converter output voltage and a negative rectified converter output voltage.
Alternatively, the rectification circuit may comprise a center-tapped rectifier in accordance with a preferred embodiment of the invention such that the rectifying element in series with the center-tap can be avoided. According to this embodiment, the center-tap is electrically connected to a negative rectified converter output voltage or the rectified converter output voltage, Vout,
As previously mentioned, the rectification circuit may comprise a voltage multiplier for example a voltage doubler circuit to increase the level of the rectified converter output voltage, Vout. The skilled person will understand that the rectifying element and/or the rectification circuit each may comprise one or more semiconductor diode(s), diode-coupled transistor(s) or synchronously controlled transistor switch(es). Each of the semiconductor diodes may comprise a MOS diode, a bipolar diode, a Schottky diode or any combination thereof.
According to one advantageous embodiment or variant of the invention discussed above with the center leg surrounded by the first and second outer legs, the isolated boost power converter comprises:
According to the latter embodiment of the invention, the first and second secondary transformer windings of the first magnetically permeable multi-legged core are electrically coupled to the rectification circuit in an indirect manner through the respective ones of the first and second secondary transformer windings of the magnetically permeable multi-legged core. The first and second secondary transformer windings are therefore coupled in series such that the rectified converter output voltage, Vout, is doubled in a symmetrical architecture or topology of transformer windings mounted on the magnetically permeable multi-legged core and the first magnetically permeable multi-legged core. The magnetically permeable multi-legged cores may be provided as separate parts, for example arranged in abutment or proximate to each other, or as an integrally formed core element which has a shared magnetically permeable structure or leg. In a preferred embodiment, the magnetically permeable multi-legged cores share a common magnetic flux path extending through a shared magnetically permeable leg. In the latter embodiment, the magnetically permeable multi-legged cores may advantageously be configured to provide magnetic flux cancellation or suppression in the shared magnetically permeable leg. This feature saves magnetic material so as to reduce material costs and size of the isolated boost power converter. The above-discussed embodiments of the present invention based on the first magnetically permeable multi-legged core and the magnetically permeable multi-legged core possess numerous favourable characteristics: scalability by the addition of further magnetically permeable multi-legged cores and associated primary side and secondary side transformer windings and drivers. This property is highly beneficial because the isolated boost power converter can readily be adapted to a whole range of applications with varying power transfer capacities. Thus, saving R&D design efforts and time, reducing design risk, reducing manufacturing costs etc. In addition, the current rating of each semiconductor switch of the first and second drivers can be halved for a given current handling capacity due to the split of input current between the first, second and possibly further drivers.
The driver control signal may comprise a PWM signal having an adjustable duty cycle, D. The adjustable duty cycle may be used to set a desired or target DC level of the rectified converter output voltage, Vout. The duty cycle is preferably set to a value between 0.5 and 1.0 after exiting or leaving a start-up state or mode, i.e. during normal boost mode operation of the isolated boost converter. The duty cycle, D, may be set or controlled in connection with a closed loop feedback control scheme for controlling any of the state variables of the isolated power converter, such as the boost inductor current or the rectified converter output voltage, Vout.
Another aspect of the invention relates to a method of generating a rectified converter output voltage, Vout, from an input voltage, Vin, by an isolated boost power converter according to any of the preceding claims. The method comprises steps of:
A preferred embodiment of the invention will be described in more detail in connection with the appended drawings, in which:
The embodiments described in detail below are particularly well-suited for power converters providing DC voltage amplification or step-up. However, the skilled person will understand that power converter in accordance with the present invention are highly useful for other types of applications both in step-up and step down voltage converting applications.
The transistor driver 106 consequently generates a primary voltage across the primary winding PW1-2 in accordance with a driver control signal, such as a pulse width modulated control signal, adapted to individually control the switching of four semiconductor switches implemented as NMOS transistors S1-S4. Each of the NMOS transistors S1-S4 is switched between conducting and non-conducting states, i.e. on-state or off-state, in accordance with an individual driver control signal applied to a gate terminal of the NMOS transistor. The isolated boost power converter 100 comprises first and second series connected secondary transformer windings, SW1 and SW2, respectively, having a center-tap or midpoint 116 arranged in-between them. The first and second series connected secondary transformer windings, SW1 and SW2, respectively, are wound around the first outer leg 132 and the second outer leg 136, respectively, i.e. separate legs of the El core 102 such that the second secondary transformer winding SW2 is arranged on the same outer leg 136 as the second half-winding PW2 of the primary transformer winding. Likewise, the first secondary transformer winding SW1 is arranged on the same outer leg 132 as the first half-winding PW1 of the primary transformer winding. A voltage transfer ratio between the primary and secondary sides of the transformer function provided by the El core 102 is set by a turns ratio, n, between the number of secondary side transformer windings relative to the number of primary side transformer windings. In the present embodiment, this turns ratio, n, equals the number of secondary transformer windings of SW1 and SW2 combined divided by the number of primary transformer windings of PW1 and PW2 combined. The turns ratio, n, may naturally vary with requirements of a particular application, in particular whether the boost converter is intended to function as a step-up or step-down converter. The turns ratio, n, is preferably set to value between 0.25 and 100 such as between 1.0 and 64. The integration of the boost inductor, Lboost, the primary transformer winding PW1-2 and the first and second series connected secondary transformer windings, SW1 and SW2 on the common or shared El core 102 is often referred to as “integrated magnetics” in the art.
A rectification circuit 118 is electrically coupled to respective outputs of the first and second secondary windings to provide a rectified converter output voltage, Vout, between rectified converter output voltage, Vout, 119 and a negative rectified converter output voltage 121. A supply capacitor C or Cout is coupled between these converter output voltages or rails to suppress ripple voltages at the output of the rectification circuit 118 and provide an energy reservoir stabilising the output voltage, Vout. In the depicted isolated boost power converter 100, the input side or primary side and secondary side are galvanically isolated by the El core 102. However, the skilled person will understand that the negative terminals or rails of the output voltage Vout and the input voltage may be electrically coupled to each other, for example through a shared GND connection without compromising the desired boost inductor discharge functionality of the first and second secondary transformer windings of the present isolated boost power converter 100. A rectifying element in form of a semiconductor diode, Df, is coupled between the center-tap 116 and the negative rectified converter output voltage 121 to facilitate a flow of discharge current through the first and second secondary transformer windings, SW1 and SW2, respectively. The discharge current subsequently flows through the rectification circuit 118 and to the output terminal or node 119 providing the rectified converter output voltage, Vout, so as to transfer energy to the output, facilitating start-up of the power converter 100 as explained in further detail below.
The transfer characteristic of the isolated boost converter is set by a duty cycle, D, of the Pulse Width Modulated (PWM) driver control signal, during normal boost operation according to:
wherein:
Vout=rectified DC output voltage of the boost converter,
Vin=the DC input voltage to the boost converter;
D=a duty cycle of the PWM control signal at each transistor input of the driver and defined as: Ton/Tperiod of a single PWM period;
n=transformer turns ratio set by the number of secondary transformer windings divided by the number of primary transformer windings.
When
the isolated boost converter may be in the start-up phase and the duty cycle, D, of the Pulse Width Modulated (PWM) driver control signal below 0.5.
When instead NMOS transistors S3, S4 and rectifying diodes D3 and D4 are conducting, a corresponding analysis applies due to symmetry. In the latter case, all transformer winding voltages and currents are reversed, which still results in Df being reverse biased by one-half of Vout. It is also worthwhile to notice that in both cases, the flux rate induced by the boost inductor, L, is not coupling to the primary and secondary transformer windings PW1-2, and SW1, SW2, respectively, of the El core 102 such that the boost inductor is magnetically decoupled from the transformer operation. Furthermore, the current in the boost inductor, L, is falling, seen as a negative voltage drop across the boost inductor in both
During the second discharge state, which can be viewed as a second subinterval of the start-up mode, the magnetic energy stored in the boost inductor, L, is discharged by circulating the stored magnetic flux through the first and second secondary transformer windings SW1, SW2. When all the NMOS transistors S1-S4 of the driver circuit 106 are turned off, i.e. non-conducting, the boost inductor current communicates to the first and second secondary transformer windings SW1, SW2 through the flyback diode Df, as shown in
Each of the center legs 734a, 734b comprises an air gap 738a, 738b which allows magnetic energy of an associated boost inductor, Lboosta, and Lboostb, respectively, to be stored therein. The isolated boost power converter 700 comprises an input terminal 104 for receipt of an input voltage, Vin, which for example may be a DC voltage between 5 Volt and 100 Volt. The first and second boost inductors, Lboosta, and Lboostb, are both coupled to the input voltage at terminal 704. A first H-bridge transistor driver 706a is coupled to the first boost inductor, Lboosta, and a second H-bridge transistor driver 706b is coupled to the second boost inductors, Lboostb. Each of the first and second three legged El cores 702a, 702b have associated primary and second transformer windings, PW1a, PW2a and SW1a, SW2a and PW1b, PW2b and SW1b, SW2b, respectively, in a topology similar to the topology discussed above in detail in connection with the first embodiment of the invention. However, only the center-tap 716 in-between the first and second series connected secondary transformer windings, SW1a and SW2a, respectively, is coupled to a rectifying element in form of semiconductor diode Df. The first and second series connected secondary transformer windings, SW1b, SW2b of the second El core 702b are not connected to a center-tap but each half-winding output is coupled in series with the corresponding secondary half-winding of the first El core 702a. The isolated boost power converter 700 comprises a shared single rectification circuit 719 coupled to respective winding outputs of the first and second secondary windings SW1b and SW2b. The rectification circuit 719 is configured as a full-wave rectifier comprising four rectifying diodes D1-D4 to produce a rectified converter output voltage Vout between positive and negative output voltage terminals or nodes 719, 721, respectively.
It is accordingly evident that the first and second primary transformer windings PW1-2a and PW1-2b are both coupled to separate drivers 706a and 706b, respectively, while the pair of first secondary transformer windings SW1a and SW1b are coupled in series between the center tap 716 and the rectification circuit 718 and the pair of second secondary transformer windings SW2a and SW2b likewise are coupled in series between the center tap 716 and the rectification circuit 718. This topology has the beneficial effect that voltage amplification, i.e. the ratio between the input voltage Vin and the rectified converter output voltage Vout, is doubled compared to the topology disclosed on
Furthermore, the first and second series connected secondary transformer windings, SW1a, SW2a are configured to discharge magnetic energy stored in the first boost inductor, Lboosta, in a manner similar to one used in the above-mentioned first embodiment of the invention by a shared magnetic flux path comprising a first magnetic flux path extending through the first center leg 734a, and the first outer leg 732a and a second magnetic flux path extending through the first center leg 734a, and the second outer leg 736a. Likewise, the first and second series connected secondary transformer windings, SW1b, SW2b of the upper El core 702b are configured to discharge magnetic energy stored in the second boost inductor, Lboostb, through a shared magnetic flux path in the upper El core 702b.
Windings on Lboost=2
# primary transformer windings, PW1-2=2
# secondary transformer windings, SW1+SW2=8
DC input voltage, Vin=25 Volt
Load resistance, RL=68.2 ohm
Core type: ELP64 available from manufacturer EPCOS AG.
Core material: N87
Air gap height (at center leg)=0.5 mm.
The graph data were acquired by maintaining Vin at 25 Volt and sweeping a duty cycle, D, of a Pulse Width Modulated (PWM) driver control signal from 0 to 0.75 and then back to 0 over a time period of 4 seconds. The time variable is indicated along the x-axis 805. The corresponding duty cycle, D, is indicated on the right-hand vertical scale 803 and the measured rectified converter output voltage, Vout, indicated on the left-hand vertical scale in Volts. The substantially linear correlation between the duty cycle and the rectified converter output voltage, Vout, is evident, and is for D>0.5 in accordance with equation (1) above. It is also noticeable that the rectified converter output voltage, Vout, is continuous across the boundaries at D=0.5 between start-up mode operation and normal boost mode operation. For D=0.75, the following electrical data were measured, input current (at Vin)=27.94 A, Vout=205.11 Volt, output current=3.006 A.
Hence, the present measurement data confirms the capability of the present experimental isolated boost power converter to start-up (D<0.5) and proceed to normal boost operation (D>0.5) in a well-behaved manner without any need for a separate flyback winding or other dedicated start-up circuitry.
Ouyang, Ziwei, Lindberg-Poulsen, Kristian, Sen, Gökan
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